Epoxy resins, due to their good processability combined with good mechanical properties and good high temperature performance, are materials commonly used for the production of high performance composite structures. Such resins usually consist of blends of various epoxies, curatives and additives, which enhance specific characteristics of the epoxy or impart new characteristics to the epoxy. These high performance epoxy matrices, reinforced with fiber, are used widely in the advanced composite industry.
Advanced composites are cured articles resulting from the combination of epoxy or other high performance matrices with a reinforcing fiber, such as glass fiber, aramid fiber (organic fiber), or graphite fiber or fabric, to form prepreg material. In order for prepregs to be useful for manufacturing primary structures, they must possess certain physical and mechanical characteristics. One such characteristic is defined by compression after impact testing, and relates to the retention of compressive strength upon sustained damage to the material. Other desirable characteristics include hot/wet performance, which relates to the performance of the material at high temperatures under humid conditions, working life, and tack and drape, which relate to processability and handleability. Therefore, these materials are evaluated by not only their mechanical performance, but also their handleability and/or processability.
Cured epoxy resins, due to high cross-linking density, display good temperature performance. Unfortunately, however, cured epoxy resin parts lack the needed toughness and are brittle. One solution used to compensate for this brittleness has been to incorporate rubber into the epoxy resin formulation. Rubber tends to improve the toughness of the epoxy by preventing the propagation of cracks and bolstering the strain capability. Other elastomers, such as nylon or liquid reactive oligomers of butadiene acrylonitrile rubbers, have also been used to enhance the toughness in epoxy resin systems.
The use of rubber or other similar elastomeric additives, however, has a significant drawback in that these additives generally possess low modulus and therefore reduce the stiffness of the cured article. The resulting composite material, therefore, displays a commensurate reduction in strength. Also, rubbers and elastomers tend to have low glass transition temperatures, which contributes to the diminished hot/wet performance of composites containing these additives. U.S. Pat. No. 4,680,076 discloses multiphase epoxy thermosets having a crosslinked continuous phase and a crosslinked discontinuous phase which contains a rubber phase.
Reactive oligomers have also been used to enhance the toughness of epoxy resin systems. The resulting systems are characterized by a phase inverted morphology. The most popular thermoplastic oligomer used to toughen epoxy resins has been an amine-terminated polysulfone. Use of this oligomer, however, presents problems in terms of reproducibility, as well as hot/wet performance and out-time. U.S. Pat. No.'s 4,656,208 and 4,656,207 disclose thermoset compositions having multiphase morphology with at least one glassy continuous phase and at least one glassy discontinuous phase, and comprising a polyepoxide component, an amine hardener, and a specified amount of an aromatic oligomer with functional groups which react with the polyepoxide and/or hardener when cured.
More recently, the industry has attempted to solve the toughness problem of epoxy composites by incorporating linear polysulfone thermoplastics into the epoxy formulation. Such polysulfone thermoplastics are specifically tailored to exhibit a spinodal morphology in the cured epoxy matrix. While these thermoplastics do help solve the tughness problem encountered with cured epoxy resins, they only marginally improve the hot/wet performance of the resin as compared to the results achievable when using rubber or elastomers as the additive.
Polyimide thermoplastics have also been used in conjunction with epoxy resins. U.S. Pat. No. 4,567,216 discloses an epoxy composition modified with a thermoplastic to improve tensile properties. The composition contains a bis(2,3-epoxyclyclopentyl)ether, a diamine hardener and a phenoxy, polycaprolactone, polyetherimide or polyarylether thermoplastic. The polyimide thermoplastic is usually combined with the epoxy in a solution blend, requiring that the amount of thermoplastic added be less than 10%. Amounts in excess of 10% polyimide thermoplastic dissolved in the epoxy formulation can not be handled due to the excessive viscosity of the material. The solution of thermoplastic in the epoxy is normally accomplished by either dissolving the two components in a common solvent and removing the solvent while mixing the material, or by heating the thermoplastic and the epoxy to a temperature above the Tg of the thermoplastic and melting the two components together. The high viscosity, as well as poor tack, drape and stiffness, make the matrix unacceptable for hot melt processing or for prepregging. Using less than 10% thermoplastic solution allows the user to control the viscosity of the matrix, however, the resultant composite will not achieve the desired toughness.
It has been found that the rate of increase in viscosity of the epoxy-polyimide thermoplastic blend is much higher than the rate of increase in toughness of the resultant matrix. In other words, the limiting factor for toughening the epoxy with the polyimide thermoplastic has been the exponential viscosity increase, making these materials unsuitable for prepregging purposes.
Therefore, while a greater percentage of polyimide thermoplastic in the resin positively increases the toughness of the resin, anything over about 10-12% polyimide increases the viscosity of the resin to a point where the resin is no longer processable. For prepreg processing purposes, the tolerable limit of addition of polyimide thermoplastic to epoxy resin is in the 8-10% range. Consequently, optimum toughness must be sacrificed in order to maintain processability of the resin.
Therefore, it is an object of the subject invention to provide a novel epoxy resin formulation for use in the advanced composite industry which displays increased toughness, and good processability and handleability, while maintaining good thermal characteristics.
It is another object of this invention to provide a process for producing a toughened epoxy matrix which displays good handleability as well as excellent performance characteristics.